The present invention is generally related to electrical test equipment and, more particularly to a system and method of identifying the location of a short circuit in an electrical conductor, such as a wire.
Many failures associated with electrical systems are due to problems in wiring or electrical connections. A common wire failing is a "short". A short occurs when the insulation between wires becomes chaffed or broken, allowing the wires the contact each other or the chassis.
Wires can be difficult to detect, especially if the wire is not accessible, or covered by other wires in the wire harness. An ohmmeter is used to measure the resistance between points, and can be used to detect if a wire is shorted. However, it is difficult to use an ohmmeter to determine the exact location of a short.
One common technique to trace the path of a wire involves the introduction of an a.c. signal to the wire under test. An inductive type sensor is then used with a meter or other indicator to locate the wire. The sensor is susceptible to fields generated in the wire. By correlating the position of the sensor and the indicator signal, the operator is able to trace the path of current flow and, therefore, the path of the wire.
Previous inductive sensor systems have used audio, or even higher frequency signals to generate the fields in the wire under test. However, the stray capacitance can couple between the wire under test, and shields, or other objects in the vicinity. This coupling of signals can be a problem with higher frequency signals and leads to unreliable field detection. While is may be easy to determine the general location of the wire under test when its field is coupled into surrounding objects, this stray capacitance makes the precise location of the wire under test difficult.
The identification of short circuits is a problem closely related to the location of wires. A shorted wire carries the test signal from the test signal transmitter to the point where it is shorted. Generally, the exact location of the short must be determined before it can be repaired.
Even when the location of a short is identifiable, accurate detection of the exact location of a short in a conductor can be difficult. Accurate detection is especially difficult when many wires are in close contact, such as in a wire bundle or wire harness. Then, some wires may radiate magnetic fields, in response to the conduction of current. Other, closely situated, wires may become susceptive to the radiated fields, and conduct current in response to the magnetic field. Thus, a short in one wire induces the appearance of a short in a neighboring wire. Likewise, signal currents may appear as shorts to the short detection equipment. Further, the resistance associated with electrical motors and light bulbs, for example, is so low that wire connected to these types of devices may appear to be shorted.
In general, large wattage light bulbs have a lower resistance than more smaller, low wattage light bulbs. Electrical motors have a higher resistance than light bulbs. Light bulbs and electrical motors are liable to give a short detector a false indication of a short circuit. Thus, if a short detector is placed closer to an electrical line connected to a light bulb, than to the wire with the actual short. The detector may falsely determine that the short is in the wire connected to the light bulb.
Other factors affect the accuracy of determining a short in a conductor. A wire that is well-shorted, having a low resistance to ground, is relatively easy to identify. A wire that is shorted with a larger resistance to ground is more difficult to detect. Also, shorts in wires associated with devices that consume large amounts of power are difficult to accurately identify. Ideally, a line with no short should also be easy to identify. That is, a line with no short should not generate a false detection of a short. However, as mentioned above, the resistance of the device connected to the wire under test in critical in the accuracy of measuring shorts.
A relative ranking of short identification, or accuracy with respect to the electrical device attached to the wire under test follows:
TABLE 1 DEVICE ACCURACY 12 volt radio &gt;90% 12 volt relay &gt;90% 12 volt, 10 watt light bulb approximately 80% 12 volt, 120 watt motor approximately 75% 12 volt, 20 watt light bulb approximately 70% 12 volt, 55 watt light bulb approximately 65%
Akira, in U.S. Pat. No. 5,621,600, discloses a portable apparatus to locate shorts in a conductor. The system uses a transmitter to generate a test signal. Current induced by the test signal through the conductor creates a magnetic field. A sensor/receiver locates the short by moving the sensor along the conductor under test, while looking for discontinuities in the field readings. While the system is effective, stray fields sometimes make the precise location of a short difficult.
It would be advantageous if a system and method were developed which used a test signal frequency and pulse width which optimized identification of the wire under test. It would be advantageous if the test signal frequency and pulse width increased the accuracy of the identification of shorted wires.
It would be advantageous if an inductive field sensor could be modified to be less susceptible to the load on a conductor, when testing the conductor for a short.
It would be advantageous if a field detector circuit could be optimally positioned with respect to a conductor under test to optimize the accuracy of readings when testing the conductor for the location of a short.
Accordingly, A system for detecting the location of a low resistance short in a conductor is provided. The system uses a transmitter to generate a test signal having a low duty cycle pulse. This transmitter has an output connected to the conductor under test, to introduce the test signal to the conductor under test.
The system also uses a hand-manipulable detector to sense magnetic fields along the conductor, induced by the transmitted test signal. This detector has an output with a test receive signal that is responsive to the transmitted test signal magnetic fields. In turn, a receiver, having an input operatively connected to the detector output of the detector, is responsive to the test receive signal. A receiver output signal, detectable by the operator, permits the detector to be operated in response to the receiver output. That is, the detector operator is able to guide the detector to the point of maximum response, as indicated by the receiver output.
The key to the new short detection system is the increased accuracy and reliability of the readings afforded with the use of a detector shield. Center shield walls, at least partially, surround the detector. An opening in the shield walls exposes the detector. The detector shield minimize the influence of magnetic fields on the detector when the shield walls are interposed between a magnetic field and the detector. The shields maximize the influence of a magnetic field on the detector when the opening is interposed between a magnetic field and the detector. In this manner, a short in a conductor is precisely located through the use of the detector shield in conjunction with the receiver output.
Besides minimizing unintended fields, the shield structure has another primary function, it optimally locates the position of the detector from the conductor under test. By maintaining a predetermined distance between the conductor and the detector, the reliability of the detector readings is further enhanced. The detector center shield walls generally have the shape of a channel, with a channel mouth. The detector center shield is shaped to locate the conductor a minimum first distance from the detector, and a maximum second distance from the detector. In this manner, the test signal transmitted from a conductor is optimally sensed when the detector shield mouth is interposed between the conductor and said detector.
The spacing is maintained with the use of a non-conductive spacer, typically plastic, interposed a first distance from the detector, to maintain a minimum first distance between the conductor and the detector. The channel shape of the shields helps insure that the wires under test are placed in the shield mouth within the second distance. The spacer prevents the wires from getting too close to the detector, further than the second distance. The first distance is approximately 17 millimeters (mm), and the second distance is approximately 42 mm.
The detector shields also include shields auxiliary to the center walls. Middle shield interior wall surfaces are located adjacent the exterior surfaces of the center walls. Likewise, outer shield interior wall surfaces are located adjacent the middle shield wall exterior surfaces. The addition shield walls increase the reliability of detector readings by minimizing the detection of fields away from the shield mouth section. The use of three, properly integrated, thin-walled shields is more efficient than the use of one or two thicker shields.
The middle and outer shield walls have a thickness in the range from approximately 0.4 to approximately 0.6 mm, while the center wall thickness is approximately 0.8 mm. Naturally, the thickness is dependent on the shield wall material. Preferably, the shield wall material is selected from the group of materials consisting of ferrous metal, aluminum, and transformer iron. Preferably, the center shield is aluminum.
A method for improving accuracy in the identification of shorts in a conductor is also provided comprising the steps of:
a) introducing an extremely low duty cycle (approximately 0.17%, or greater), extremely low frequency (approximately 2 Hz), pulse transmitter test signal to the conductor under test, whereby the conductor radiates a field in response to the test signal; PA1 b) while maintaining an optimal separation between the detector and the conductor under test, detecting a plurality of fields, radiated by the transmitter test signal in the conductor under test, in response to a corresponding plurality of positions along the conductor. A detector shield minimizes the influence of fields on the detector when the shield walls are interposed between a field and the detector, and maximizes the influence of a magnetic field on the detector when the opening is interposed between a magnetic field and the detector; PA1 c) comparing the plurality of fields detected in Step b) to determine a change in detected fields; and PA1 d) establishing a correspondence between change of field detected in Step c) and a position along the conductor under test, whereby the change of field indicates a short in the conductor.
As described above, a non-conductive spacer is interposed between the detector and the detector shield mouth, and Step b) includes interposing the spacer between the conductor under test and the detector, whereby an optimal spacing is maintained for the sensing of test signals on the conductor.